Asynchronous master-slave print system employing charging and discharging of a capacitor

ABSTRACT

A master-slave print system in which the master station has a code carrier and each slave station has a type carrier, all the carriers moving at substantially the same velocity. Each character transmitted from the master to a slave station is represented in a transmitted signal by the time interval between a pair of pulses and each such time interval is indicative of the position of the character on the master&#39;&#39;s code carrier with respect to a reference point thereon. A capacitor at the slave station starts charging at the beginning of the transmitted time interval. When the time interval ends or when a reference point on the slave&#39;&#39;s carrier is detected (whichever occurs first), the charging operation is arrested and the charge level held. When the second of the latter two events (reference point detection or time interval termination) occurs, the capacitor is discharged at the same rate it was charged and when it reaches its initial reference level the slave print hammer is fired to print the transmitted character.

United States Patent [72] Inventors Frederick F. Ladd, Jr. 3,185,963 5/1965 Peterson et al. 178/69.5X Newbury, Mass.; 3,191,158 6/1965 Sherman 320/1X David L. Nettleton, Moorestown, NJ. 3,208,050 9/1965 Bird et al. 178/69.5X [21] Appl. No. 754,021 3,215,778 11/1965 Abramson 197/49X [22] Filed Aug. 20, 1968 3,243,665 3/1966 Fayer et al. 101/93X [45] Patented Jan. 12,1971 [73] Assignee Mohawk I )ata Sciences Corporation Burr East Herklmer, N.Y.I a corporation of New York 54 1 ASYNCHRONOUS MASTERSLAVE PRINT ABSTRACT: A master-slave print system in which the master SYSTEM EMPLOYING CHARGING AND station has a code carrier and each slave station has a type car- DKSCHARGING OF A CAPACITOR ner, all the carriers moving at substantially the same veloc ty. 15 Claims, 6 Drawing Figs Each character transmitted from the master to a slave statlon 1S represented in a transmitted signal by the time interval U-S. t r t between a pair ofpuises and each uch time interval is indica- 101/931 197/491 330/ l tive of the position of the character on the master's code carri- [5 l 1 Int. er e t to a reference p int th A p it t h l Field of Search 101/93; slave station starts charging at the beginning of the transmitted 178/69.5, 35, 41; 320/1; 197/55, 49 time interval. When the time interval ends or when a reference int on the slave s carrier is detected (whichever occurs [56] UNITE :;Z $S ENTS t sst), the charging operation is arrested and the charge level held. When the second of the latter two events (reference 1,940,016 12/ 1933 Ranger 178/ 69.5 point detection or time interval termination) occurs, the 2,336,929 12/1943 Doyle 320/1X capacitor is discharged at the same rate it was charged and 2,680,808 6/ 1954 Nolde 320/ 1X when it reaches its initial reference level the slave print 3,088,064 4/ 1963 Anger 320/1 hammer is fired to print the transmitted character.

2 C II. SYNC 8 E 22 26 I T R i i 24 8 RESET I'LPRINT g l2 COMPARATOR R GATE o 32 FF n' A 's R 40 INPUT 28 ME E EE P'L SLAVE STATION ss 11 HOME lOus CONTROL CIRCUIT v IOI PRINT FIRE TO ADDITIONAL SLAVE STATIONS PATENTEDJA'NI2BYI N 3555183 suwaura I v l: r: J=

SYNC

PRINT HOME CAP c DIFF AMP

FIRE

ANDISS FIGB PATENTEU JANI 2am sum 1; or e v v C mm52 E mm; C v q 7 ASYNCHRONOUS MASTER-SLAVE PRINT SYSTEM EMPLOYING CHARGING AND DISCHARGING OF A CAPACITOR BACKGROUND OF THE INVENTION This invention relates to asynchronous print control systems, and more particularly, to a master-remote slave print system wherein information generated at the master station is automatically duplicated at the slave stations.

Many high-speed low cost printing techniques have emerged during the past 20 years and in virtually all of these techniques the principal of on-the-fly printing is employed wherein a type carrier is constantly driven at a high velocity past a print hammer station and a hammer is fired at a precisely controlled time to momentarily force a print medium against a moving type carrier to print the selected character. This technique has produced printing at the rate of up to 30 characters per second per hammer station.

Naturally, at these high print speeds hammer control must be very precise since mistiming on the order of only 20 or 30 microseconds (us.) in the operation of the hammer can cause misregistration of the print impression and, at times, even illegibility.

Owing to these rigid timing requirements remote high-speed print stations capable of receiving and printing transmitted data have had to incorporate their own complete timing controls for synchronizing the printout of data with the movement of the local type carrier. This requirement has prevented the introduction of a truly low cost high-speed remote print ter minal capable of deriving timing as well as printout data from a master station.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an improved high-speed print station capable of being slaved to a master station and of operating upon both printout and timing data transferred from the master station.

Another object is to provide a plurality of remotely located high-speed print stations controlled by a single centrally located digital comparator.

Still another object is to provide an improved system where a plurality of on-the-fly printers receive information embodied in the form of signal modulations delineating time intervals and print the same data therefrom even though synchronization is not present between the respective moving type carriers of the printers. A further object is to provide an improved onthe-fly printer operable on data signals encoded in terms of the position of the printer type carrier without requiring a feedback link between the printer and the data source.

In accordance with the invention, the master station and all remote slave print stations are provided with moving character-bearing members each having a home reference point thereon and operating at substantially the same velocity. Data is transmitted in the form of time intervals between transmitted pulses. Each time interval represents the position of a transmitted character on the masters character-bearing member with respect to its home reference point. In the remote stations, each transmitted time interval is decoded for rinting by a tinting circuit which is set into operation at a measured rate at the beginning of each time interval and which is stopped in response to the occurrence of the end of the received time interval or the passage of the home reference point on the local type carrier, whichever occurs first. When the last-to-occur of the latter two events takes place, the timer is driven back to its start position at the measured rate and when that position is reached the print hammer is tired to print the transmitted character.

The timer thus acts as a form of analogue buffer storage unit for storing the transmitted time interval until the type carrier at the remote printer arrives at the proper reference position. in other words, the slave station receives the time interval data transmitted on the time base of the master station and converts it to the time base of the slave station whereupon it is utilized to print the desired data. This thus permits asynchronous operation between the master and slave stations without requiring transmission of digital data between the stations and without the need for a complete timing, comparing and control system at each station.

These and other objects, features and advantages of the invention will be made apparent by the following detailed description of a preferred embodiment of the invention, the description being supplemented by drawings as follows:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram divided by a dashed line into two sections. The top section illustrates a master station receiving a digital input representing a character to be printed and transmitting a signal modulated to represent the character in terms of a time interval. The bottom section illustrates a slave station operating on the transmitted signal to print the character represented thereby.

FIG. 2 is a schematic diagram illustrating in detail the slave station control circuit 80 schematically shown in FIG. 1.

FIG. 3 is a waveform diagram depicting operation of the circuit of FIG. 2 under conditions of asynchronism wherein the transmitted time interval and the time interval reconstructed at the slave station overlap one another.

FIG. 4 is a waveform diagram depicting operation of the circuit of FIG. 2 under conditions of asynchronism wherein the transmitted time interval and the time interval reconstructed at the slave station do not overlap one another.

FIG. 5 is a waveform diagram depicting operation of the cir-- cuit of FIG. 2 under conditions of asynchronism wherein the termination of the transmitted time interval exactly coincides with the initiation of the time interval reconstructed at the slave station.

FIG. 6 is a waveform diagram depicting operation of the circuit of FIG. 2 under conditions of synchronism.

DETAILED DESCRIPTION FIG. 1 schematically depicts a system in accordance with the principals of the invention and shows a master station and one slave station coupled thereto, although additional identical slave stations may be connected into the system. The

master station is shown in the top half of the FIG. above the dashed line and comprises a synchronous motor l0 operating from an AC power supply 34 to drive a code wheel 12 at a constant velocity past a seven channel magnetic pickup head 14. The wheel 12 rotates counterclockwise as shown by the arrow and has inscribed about its periphery a series of magnetically encoded seven-bit characters, such as A3, representing a complete type font. For purposes of illustration, the embodiment herein described is shown as having only alphabetic characters, 26 in number, but, as may be readily appreciated, anysize type font may be employed. The 26 coded characters are distributed at equal intervals about wheel 12 and collectively occupy most of the periphery. The characters are spaced apart from one another by amounts exactly equaling the distances between type characters on the type wheel of the slave print station described subsequently. A 27th uniquely coded character 13 is included on wheel 12 just before the first character of the font and is spaced therefrom by a distance equaling the distance between the various font characters. This 27th character is a sync character and is used to generate a SYNC pulse which is employed to establish the time base for all printers in the system. After the last code character on wheel I2, a gap is provided for timing purposes described subsequently.

Thus, as the wheel 12 rotates, each master station timing cycle commences when the sync character passes the pickup head I4. The sync character is recognized by a decode circuit 116 and a SYNC pulse approximately 10 microseconds (us) wide is generated by circuit I6 and fed to a transmitter 26 for transmission on line 4-0. Each SYNC pulse is also fed to the input end of a shift register 2. After the sync character passes the pickup head 14 the various font characters thereafter pass head 14 at regular intervals and after the last character has passed, the empty portion of the code wheel passes head 14 prior to reappearance of the sync character and initiation of the second revolution and timing cycle. As pickup head 14 detects each of the coded font characters, signals representative thereof are sequentially transmitted over a seven-line cable 36 to a digital comparator 18.

Comparator 18 also receives, via seven-line input cable 32, a seven-bit input character representing a character to be printed. A gate circuit 30 presents the input character to comparator 18 during every third revolution of code wheel 12. When a code wheel character appearing on cable 36 is detected by the comparator to be equal to the input character a PRINT signal is generated at the output of the comparator and sent to transmitter 26 for transmission on the line 40.

Shift register 20, AND circuit 22, delay line 24 and bistable multivibrator (flip-flop) 28 are provided to control gate 30. Shift register 20 has three stages, the leftmost of which receives the SYNC pulse as an input once per revolution of wheel 12. Every third SYNC pulse that is generated fills the shift register (each stage thereof is in the one state) so that all three inputs to AND 22 become positive. This causes a positive output signal to issue from AND 22 whereby flip-flop 28 is set and shift register 20 is reset to the all zero condition. The setting of flip-flop 28 opens gate 30 whereby the coded character on input cable 32 is presented to the comparator.

Delay line 24, which has a delay equal to the time it takes the full font of characters to pass pickup head 14, subsoquently presents the output pulse from AND 22 to the reset input of flip-flop 28, causing the flip-flop to become reset whereby gate 30 closes. Thereafter, the next ensuing SYNC pulse places shift register 20 in the 100 condition and, since AND 22 is not enabled by this combination of inputs, flip-flop 28 remains reset during that revolution of wheel 12 and the input character is not presented to the comparator. At the beginning of the next revolution of wheel 12 the shift register is placed in the 110 condition and as with the previous state of the shift register, AND 22 is not enabled and gate 30 remains closed for that revolution of wheel 12 also. At the beginning of the next revolution, however, the SYNC pulse causes shift register 20 to shift to the 111 state, bringing about the opening of gate 30 and the presentation of the input character in the manner previously described.

It is therefore seen that a PRINT pulse issues from comparator 18 every third revolution of wheel 12. The reason for this utilization of only one-third of the revolutions of code wheel 12 for printing is to permit adequate time for record feeding to take place at the slave print stations, and also to allow sufficient time for the slave printer to operate properly even under the worse conditions of nonsynchronism.

For purposes of illustration the system is assumed to be capable of printing at the rate of characters per second. Since a PRINT pulse is generated only once every three revolutions of the code wheel, the latter must be driven at an angular velocity of 30 revolutions per second or 1800 rpm. Each revolution of the code wheel thus takes 33% milliseconds (ms). Of this period of time it is assumed that the 27 code characters on the wheel pass the pickup head 14 during the first 30 ms of the revolution and that the remaining 3% ms of the revolution is dwell time (empty space on the wheel). The time between characters is thus,l.l54 ms. Thus, the code character representing the first character (e.g. A) of the type font passes pickup head 14 1.154 ms after passage thereby of the sync character. The second character of the font passes the head 2.308 ms after the sync character, etc., until the 26th and last character of the font passes the head 30 ms after the sync character.

The slave station receiving the SYNC and PRINT signals on transmission line 40 for printing of the characters represented thereby is shown in the lower half of FIG. 1. A type wheel 52 is constantly rotated counterclockwise by a synchronous motor 10' identical to the motor 10 at the master station. As shown,

the motor 10 operates from the same AC power supply 34 used to operate the motor 10 so that the angular velocity of the output shafts of the motors is substantially identical. Type wheel 52 has the same diameter as code wheel 12 and has the 26 characters of the type font distributed about its periphery in the same sequence as the code characters ii? on code wheel 12. There is no character on type wheel 52 corresponding to the sync character on wheel 12 and, of course, there is a blank portion on the periphery of wheel 52 corresponding to the empty portion of the periphery of wheel 12.

While motors l0 and 10' are shown physically wired to the same power supply, it is not necessary that this actually occur in practice. Instead, remote power supplies operated at the same frequency will suffice to insure that the output velocity of the two synchronous motors are substantially identical at least over the period of one revolution (33.3 ms). This is not a difficult requirement to obtain in practice, keeping in mind that slight constant offsets in the output velocities of the two motors can be compensated for by circuit adjustments at the individual motors.

Type wheel 52 has on one side a magnetically inscribed bit 56 which passes a magnetic pickup head 54 once each revolution of the type wheel. The signal thus generated by head 54 is amplified by an amplifier and pulse shaper 58 and fed to the input of a single-shot multivibrator 60 which generates a 10 microsecond HOME output pulse in response thereto. This HOME pulse is transmitted to a control circuit along with the SYNC and PRINT signals received on transmission line 40. Control circuit 80 generates a FIRE pulse or a PRINT pulse at its output which is fed to a driver circuit 66 which powers a solenoid 64 to operate a print hammer 62. Hammer 62 drives a record medium 70 and ink ribbon (not shown) against the rotating type wheel 52 to cause printing of the selected character on the medium 70. After the printing operation takes place the record medium is incremented to the left to prepare for the next printing cycle.

While a SYNC pulse is transmitted on line 40 each revolution of code wheel 12, printing at the slave station occurs only when a SYNC pulse is followed by a PRINT pulse. Each such pair of transmitted pulses delineates a time interval which is a representation, timewise, of the position on code wheel 12 of the input character appearing on input cable 32. As hereinafter described, the slave station takes this transmitted time interval and reconstructs it on a time base synchronized to the rotation of type wheel 52. The HOME signal feeding into control circuit 80 bears the same relationship to the rotation of wheel 52 that the SYNC pulse bears with respect to the wheel 12. However, since print hammer 62 cannot act instantaneously owing to the mechanical parameters which govern its operation, it can be seen that bit mark 56 will not have the same position on wheel 52 that the SYNC character has on wheel 12 with respect to the font characters. Bit mark 56 will be offset by an amount sufficient to compensate for the mechanical delays inherent in the operation of the hammer.

With reference to FIG. 2 the control circuit 80 is hereinafter described in detail. The signal on transmission line at) is fed to a receiver and the receiver demodulates the signal and provides at its two output lines SYNC and PRINT pulses corresponding to the SYNC and PRINT pulses embodied in the transmitted signal.

The transmitted time interval defined by the SYNC and PRINT signals is reconstructed on the proper time base by a timing circuit operating on a capacitor charge-discharge principal. A capacitor C is connected to a linear charging circuit including transistors T1 and T2, diode D3, and resistors Rli and R2.

Charging current is supplied to capacitor C at a linear rate from a reference voltage source +V when a positive voltage level is applied to the base of transistor T2 causing the transistor to switch to its nonconducting state whereby transistor T1 is biased into conduction by a reference potential +V, supplied through diode D3. Current thus flows through resistor R2 to charge capacitor C. During this charging operation transistor T3 is biased into its nonconducting state.

When the charging operation is terminated by a negative voltage transition at the base of T2, which turns that transistor and accordingly biases transistor T1 off, capacitor C mainrains its charge level substantially without change for a relatively long period of time (at least a few milliseconds).

A linear rate discharging circuit including transistors T3 and T4, diodes D1 and D2 and resistor R3 is provided for discharging the capacitor at the same rate that it was charged. When a negative voltage level is presented to the base of transistor T4, switching that transistor off, transistor T3 is biased into conduction by a V, voltage level supplied to the base of T3 through diode D2. This causes capacitor C to discharge through T3 and resistor R3 to the negative reference voltage source V,. So long as transistor T4 is held off, the discharge operation will continue at the constant rate and when the capacitor is fully discharged (reaches the ground voltage level to which its one terminal is tied) it remains in that state so long T3 is held on by T4.

A differential amplifier 165 is connected across capacitor C and is designed such that its output polarity switches from a negative level to a positive level as soon as a charge begins to accumulate on capacitor C. When the capacitor is discharged back to ground, the output of amplifier 165 reverses itself, switching from the positive level back to the negative level.

The output from amplifier 165 is inverted by an inverter 167 and transmitted to the input of a single-shot multivibrator 169 which operates in response to a positive going transition at its input to produce a ten microsecond wide output FIRE pulse. This output pulse is transmitted to the hammer driver circuit 66 (FIG. 1) to operate the print hammer 62.

A rapid discharge circuit including a transistor T5 and diode D4 is provided to enable rapid discharge of capacitor C under certain circumstances; Whenever transistor T5 is biased into conduction by the application of a positive voltage to its base, a direct low impedance discharge path is provided across the capacitor to substantially instantaneously discharge it to its reference (ground) level.

On-off control of the charging circuit is effected by a control circuit including a flip-flop 135 an OR circuit 133, AND circuits 131 and 137 and a delay line 139. AND 137 receives its inputs from the set output of flip-flop 135 and from the reset output of a flip-flop 115. Whenever both of these inputs are positive (flip-flop 135 is in its set state and flip-flop 115 is in its reset state) AND 137 applies a positive voltage level to the base of T2, turning T1 on and initiating a charge operation. Resetting of flip-flop 135 through presentation of a HOME or PRINT signal or both to OR 133 resets flip-flop 135, causing the set output thereof to shift negative whereby AND 137 is disabled and the charging operation terminates through the turning off of T1. The next ensuing SYNC pulse again enables AND 131, setting flip-flop 135 and initiating a new charge operation. An inverter 107 is provided to inhibit AND 131 under certain conditions to be described subsequently.

Control of the capacitor discharge circuit T3, T4 is effected by a logic circuit including a flip-flop 127, AND circuits 119, 125 and 129 and an OR circuit 123. Each SYNC pulse gated through AND 125 operates to set flip-flop 127 whereby a positive output issues from AND 129 (when flip-flop 115 is in its reset state). The positive level at the output of AND 129 turns transistor T4 on whereby T3 is turned off, disabling the discharge circuit. Resetting of flip-flop 127 from the output of OR circuit 123 disables AND 129 and turns the discharge circuit back on. An output pulse is generated by OR 123 in response to any one or more of three different inputs. The first input comes from AND 119 and exists whenever a pulse issues from OR 133 after flip-flop 135 has been reset. The delay provided by delay line 139 is on the order of to 12 microseconds and operates to prevent the same pulse that resets flip-flop 135 from also resetting flip-flop 127. The reason for this is made apparent in the subsequent description of circuit operation.

The second input to OR 123 comes from an AND circuit 117 which supplies a positive pulse whenever the HOME and PRINT pulses coincide. The third input to OR 123 is generated by an AND circuit 155 whenever an output signal is generated by single-shot 149 at a time when flip-flop 143 exists in its set state. The output pulse from AND 155 signifies the absence of a PRINT pulse in the period of time between two SYNC pulses.

As with flip-flop 135, flip-flop 127 is held in its reset state by the output of inverter 107 which inhibits AND gate whenever the output from an AND gate 161 is positive. This latter condition occurs during the coincidence of a SYNC and a HOME pulse. The existence of this condition signifies that the master print station and the slave print station are operating in synchronism with one another. Whenever this chance condition prevails, any ensuing PRINT pulse need only be gated directly through to the hammer driver to operate print hammer 62 for correct printing. Therefore, the output from AND 161 sets a flip-flop 163, providing a positive enabling voltage level to AND 102 and a negative disabling voltage level to AND circuit 103 through an inverter 105. Any FIRE pulse which may be generated by single-shot 169 while the circuits are thus conditioned is ineffective to cause printing and any PRINT pulse which appears on the transmission line 40 during this time is directed through AND gate 102 and OR 101 to the hammer driver.

Flip-flop 115 controls AND gates 129 and 137 whereby these ANDs are deconditioned each time flip-flop 115 is set by an output from OR circuit 121 and are reconditioned when flip-flop 115 is reset by an output from AND 113. OR 121 sets the flip-flop in response to an output from AND 119 or AND 117. AND 119, as previously explained, generates an output pulse in response to the last to occur of the pair of HOME and PRINT pulses generated during a print cycle. AND 117 generates an output in response to coincidence of the HOME and PRINT pulses. The output from OR 121 also resets flipflop 111 which is set by each SYNC pulse fed through a delay line 109. The delay of delay line 109 is set on the order of 10 to 12 microseconds, so that after theoccurence of an output from OR 121 flip-flop 115 will stay set through the next SYNC pulse and will be reset when the second SYNC pulse enables AND circuit 113. This then reconditions ANDs 129 and 137.

The purpose of flip-flop 115 is to make sure that the charging circuit is held off and the discharging circuit is held on a sufficient time to permit complete discharge of capacitor C. If this circuit was not provided there would be some instances when the next SYNC pulse would occur prior to the complete discharge of the capacitor and this would result in erroneous operation if the SYNC pulse was allowed to perform its usual function of turning the charging circuit on and turning the discharge circuit off.

Since the transmitting circuit, previously described, operates to preclude transmission of a PRINT pulse two out of every three revolutions of the code wheel and since there are times in the operation of the system when no printing occurs and thus no PRINT pulses are transmitted for a long period of time, control circuits are provided at the slave station for monitoring the interval of time following each received SYNC pulse and for producing an output indication when it is ascertained that there will be no print pulse following that SYNC pulse. This indication is used to turn on the fast discharge circuit to return capacitor C to its ground reference level in time for the next SYNC pulse which initiates a new charge operation. A flip-flop 143 is set by the SYNC pulse whereupon the set output of the flip-flop conditions and AND gate 155 and initiates operation of a single-shot 145. The period of the single-shot is 30.5 milliseconds which, as previously described, is the period of time required for all of the coded font characters to pass the pickup head 14 at the master station. When singleshot 145 times out, the negative-going transition presented to inverter 147 is converted thereby to a positive-going transition which initiates operation of single-shot 149). The 10- microsecond pulse issuing from this single-shot passes through AND gate to reset flip-flop 143 if no PRlNT pulse had occurred during the period of singleshot 145 to of flip-flop 163. The negative output level thus produced at the output of inverter 157 is transmitted to AND gate 103 and inhibits operation thereof during the rapid discharge operation so that the FIRE pulse generated at the end of that operating is not misinterpreted by the system as a true FIRE pulse.

Resetting of flipflop 143 by OR 141 restores the no print" monitoring circuit so that it is ready in time for the next SYNC pulse. OR 141 operates in response to PRINT pulses, FIRE pulses and outputs from AND 155. Thus, since at least one of these pulses occurs during each SYNC interval, the monitoring circuit is always operative.

OPERATION With reference to FIGS. 2 and 3 a first print operation is hereinafter described. FIG. 3 shows the sequence of events occuring during the time period spanned by four consecutive SYNC pulses transmitted from the master station. A PRINT pulse follows the first SYNC pulse signifying the 26th (i.e. Z) character in the type font. In accordance with the operation of the transmission control circuits, the second and third SYNC pulses are not followed by a PRINT pulse. Thus, the print interval transmitted by the master station is defined by the interval S1. The slave print interval which is generated by the control circuit of FIG. 2 to cause printing of the selected character at the slave station is identified by the interval S2. Of course, S1 equals 52.

It is to be noted that the stateof synchronization between the master and slave stations depicted in FIG. 3 is one where the HOME pulses slightly lag the SYNC pulses. Just prior to receipt of the first SYNC pulse by receiver 100 flip-flops I27, 135, 115, 143 and 163 are in the reset state and flip-flop 1 11 is in the set state.

The first SYNC pulse sets flip-flops 127, 135 and 143. The setting of flip-flop 135 turns on the charging circuit T1 and the setting of the flip-flop 127 turns off the discharging circuit T3. The voltage across capacitor C begins to rise at a constant rate and the output of differential amplifier 165 switches positive. The setting of flip-flop 143 partially conditions AND circuit 155 and initiates single-shot 145, thereby setting the no print" monitoring circuit into operation.

Next, the HOME pulse generated by the type wheel 52 is received by the control circuits. This pulse feeds through OR circuit 133 to reset flip-flop 135, whereupon the charging circuit T1 is turned off to arrest the voltage rise in capacitor C and causing the potential thereacross to hold. The resetting of flip-flop 135 by the HOME pulse does not cause an output to issue from AND 119 since delay line 139 prevents the positive transition at the reset output of the flip-flop from being presented to the input of AND 119 during the time OR 133 is responding to the HOME pulse.

When the PRINT pulse is received, it feeds through OR circuits 133 and 141. The output from OR 133 passes through the now-enabled AND gate 119, resetting flip-flop 127 and setting flip-flop I 15. This turns on the discharge circuit T3 and disables AND gates 129 and 137 so that the state of the charge-discharge circuits is not disturbed by the next incoming SYNC pulse. The output from OR 141 resets flip-flop 143 so that AND gate 155 is disabled. This prevents the subsequent output from single-shot 149 from generating an output from AND 155.

After the charge circuit T3: has been turned on the voltage across capacitor C decreases at the same rate at which it had previously risen and when it reaches the ground reference level the output from differential amplifier 165 switches negative causing a FIRE pulse to be generated by single-shot 169. This pulse feeds through AND gate 103 and OR gate 101 to the hammer driver to set print hammer 62 in motion. Some predetermined length of time thereafter the hammer strikes record medium 70 (FIG. 1) against type wheel 52 to print the selected character Z.

When the second SYNC pulse is received, it sets flip-flops 127 and 135 but this action has no effect on the chargedischarge circuit since AND gates I29 and 137 are blocked by flip-flop 115. This flip-flop had been set by the output from OR 121 during the previous cycle, the same output having reset flip-flop 111. Since the delay line M19 prevents the second received SYNC pulse from setting flip-flop III in time to gate a pulse through AND 113, flip-flop is not reset by the second SYNC pulse. However, this pulse does set flip-flop 143 to initiate the no print" monitoring circuits.

Since no PRINT pulse is received following the second SYNC pulse, single-shot 145 times out while flip-flop 143 is still in the set state, thus causing an output to appear at AND 155. The output from AND 155 turns on the rapid discharge circuit T5 but since there is no charge on capacitor C this has no effect.

When the third SYNC pulse is received, it feeds directly through AND 113 to reset flip-flop 115 whereby AND gates 129 and 137 are reconditioned. This SYNC pulse also feeds through AND gates 131 and to set the flip-flops 135 and 127 whereby the charging circuit T1 is once again turned on and the discharging circuit T3 is turned off. This initiates another charge buildup on capacitor C, switching the output of differential amplifier 165 to its positive state.

The cycle thereafter proceeds in the same manner as the first cycle with the HOME pulse acting to turn off the charge circuit so that the voltage level across capacitor C remains stable. However, since no PRINT signal is received during the cycle, flip-flop 143 is still in its set state when single-shot 149 times out and an output is thus generated from AND 155. This output resets flip-flop 127 to turn on the discharge circuit T3 and to simultaneously turn on the rapid discharge circuit T5 whereby the voltage level on capacitor C is quickly reduced to the ground reference level. This, of course, causes the output of the differential amplifier 165 to switch back to its negative state and generate a FIRE pulse. This pulse resets flip-flop 143 to print the no print monitoring circuit for the next cycle but is blocked from passage through AND 103 since the output from AND has caused the inverter 157 to go negative, temporarily inhibiting AND 103. This, of course, is a desired action since the FIRE pulse does not denote a true print cycle.

The operation of the control circuit continues in the manner just described until another PRINT pulse appears at the input to initiate another print operation.

Referring now to FIG. 4, the operation of the control circuit is hereinafter described wherein the condition of synchronism between the master and slave stations is one where the HOME pulses slightly lead the SYNC pulses. For illustration, the circuit is described as operating to print the same character 2. as above. It can be noted from FIG. 4 that the operative pulses occur in the sequence SYNC, PRINT, HOME. The transmitted print interval 51 therefore terminates before the beginning of the slave print interval S2 and the period of delay between the beginning of the transmitted print interval and the end of the slave print interval is substantially greater than in the previously described condition of synchronism where the HOME pulse lag the SYNC pulses.

Just before the first SYNC pulse is received, the condition of the circuit is the same as previously described. As before, the SYNC pulse turns on the charging circuit T1 and turns off the discharging circuit T3, so that the voltage across capacitor C begins to rise at the predetermined rate. As shown in FIG. 41, This voltage rise continuous throughout the full extent of the transmitted print interval and is not terminated until the PRINT pulse occurs to reset flip-flop 135 to turn off the charging circuit. The voltage across capacitor C remains stable until the HOME pulse acts to reset flip-flop 127, turning the discharging circuit T3 on.

The purpose for flip-flop 115 now becomes readily apparent since if AND gates 129 and 137 were not inhibited at the time the charging operation began, the second SYNC pulse, in setting flip-flops 127 and 135, would interrupt the discharging operation and erroneous printing or no printing at all would be the result.

When the capacitor has been fully discharged differential amplifier 165 switches negative and a FIRE pulse is transmitted by single-shot 169 to operate the print hammer, printing the selected character.

When the next SYNC pulse comes in, the usual events occur, i.e., the charging circuit is turned on, the discharging circuit is turned off, capacitor C begins to charge at the mea sured rate and the differential amplifier switches positive. Since no PRINT pulse follows the SYNC pulse AND 155 generates its end of cycle output to cause rapid discharge of the capacitor and to prepare the circuit for the next cycle.

FIG. illustrates the sequence of operation that ensues when the PRINT and HOME pulses exactly coincide. As is apparent, the first SYNC pulse initiates the usual series of events. However, since the next event is the coincident occurence of both the PRINT and HOME pulses AND 119 cannot perform its usual function of resetting flip-flop 127. Thus AND 117 is provided to take care of this function when the PRINT and HOME pulses coincide. The output from AND 117 feeds through OR 123 to reset flip-flop 127 and through OR 121 to set flip-flop 115. The effect of this, as can be seen from FIG. 5, is simultaneous turning off of the charging circuit T1 and turning on of the discharging circuit T3. Thus, capacitor C starts discharging as soon as it stops charging and the transmitted and slave print intervals S1 and S2 are rendered contiguous with one another. The remaining operations of the circuit are the same as previously described in connection with FIG. 4.

FIG. 6 shows the operation of the circuit under conditions of exact synchronism between the master and slave stations (coincidence of S1 and S2). The same conditions that exist to cause asynchronous operation, it will be appreciated, also act to cause random periods when the SYNC and HOME pulses coincide. The circuit is designed so that when this condition is detected to exist, the charging and discharging circuits are rendered inoperative and the PRINT pulse is channeled directly through AND 102 and OR 101 to the hammer driver. Exact coincidence between the HOME and SYNC pulses is detected by AND circuit 161.

The output from AND 161 is fed to inverter 107 and the output therefrom deconditions the AND gates 125 and 131 so that the SYNC pulse cannot act to set the flip-flops 127 and 135. Thus, so long as synchronism exists between the HOME and SYNC pulses, the charging circuit T1 remains turned off and the discharging circuit T3 remains turned on whereby capacitor C is held clamped to its ground reference.

The output from AND 161 also sets the flip-flop 163. The set output from the flip-flop is fed to inverter 105 and the output therefrom deconditions AND 103 so that any FIRE pulses that might be generated by single-shot 169 are blocked. Additionally, the set output of flip-flop 163 opens AND gate 102 to permit any PRINT pulses received at the input to be channeled directly to OR 101 and to the hammer driver.

The only portion of the control circuit that continues to operate as usual is the no print monitoring circuit. Each SYNC pulse sets flip-flop 143 to initiate operation of singleshot 145. If no PRINT pulse occurs during the cycle an output issues from AND 155 and is fed to the input of OR 141 to reset flip-flop 143, preparing the circuit to monitor the next cycle. The output from AND 155 also acts to reset flip-flop 163 whereupon AND 103 is restored to its partially conditioned state and AND 102 is blocked. This restoration of the AND gates 102 and 103 to their normal condition is necessary since the circuit must be prepared to accept the next print cycle under conditions of nonsynchronism, should such occur.

It will be appreciated that various changes in the form of the details of the above described preferred embodiments may be effected by persons of ordinary skill without departing from the true spirit and scope of the invention.

We claim:

1. A system for controlling a printer including a moving type carrier, a record medium adjacent said carrier and a print hammer selectively operable to drive said carrier and said record medium together to print a character, said system comprising:

a circuit for generating a character-representing signal, said circuit including means for generating SYNC pulses. at least some of which are followed by u PRINT pulse, the time interval between a SYNC and a following PRINT pulse representing a character with different time intcrvals representing different characters;

means for generating a HOME pulse each time a full type font on said carrier passes said hammer;

a capacitor;

means for charging said capacitor at a predetermined constant rate;

means responsive to a SYNC pulse to initiate charging of said capacitor at said constant rate;

means response to the first-to-occur of the HOME and PRINT pulses immediately following said SYNC pulse for arresting said charging operation;

means for maintaining the amount of charge on said capacitor substantially constant at the value of charge which said capacitor had at the time charging was arrested;

means for discharging said capacitor at said predetermined constant rate;

means responsive to the last-to-occur of said HOME and PRINT pulses for rendering said means for maintaining ineffective and initiating discharge of said capacitor; and

means operable upon completion of said discharge to operate said hammer to print a character.

2. The system set forth in claim 1 wherein said type carrier moves at a substantially constant velocity.

3. The system set forth in claim 2 wherein said type carrier comprises a constantly rotating type wheel.

4. The system set forth in claim 1 wherein said circuit means comprises:

a moving code carrier driven at substantially the same velocity as said type carrier and having coded indicia thereon, a plurality of said indicia representing the different characters on said type carrier and being spaced apart a distance corresponding to the distance between characters on said type carrier another of said indicia located at a reference point on said code carrier;

means for sensing said indicia;

decode means connected to said sensing means for generating a SYNC pulse when said reference indicia is sensed by said sensing means;

means for supplying a coded input character; and

comparison means connected to said sensing means and said input means for generating a PRINT pulse when said input character matches one of said character-representing indicia.

5. The system set forth in claim 4 wherein said circuit means is located remote from said type carrier and is connected to the remainder of said control system by a transmission line.

6. The system set forth in claim 4 wherein said code carrier and said type carrier are driven by synchronous motors connected in the same power supply network.

7. A system for controlling a printer wherein type characters in a font on a moving type carrier are selectively im printed on a record medium adjacent the carrier, the system comprising:

a circuit for generating a signal of variable time interval for representing characters to be printed, different characters being represented by signals having different time intervals;

means for generating a HOME pulse each time a full type font on said carrier passes said record medium;

reversible timing means;

means for initiating operation of said timing means in one direction at a constant rate in response to the beginning of a said signal of variable time interval;

means for stopping operation of said timing means in response to the first-to-occur of the end of said signal of variable time interval or a HOME pulse;

means for maintaining said timing means in the condition it was in at the time it was stopped;

1 1 means for operating said timing means in a reverse direction at said constant rate such that it returns to its original state in response to the last-to-occur of a HOME pulse or the end of said signal of variable time interval; and means operable in response to said timing means returning to its original state for printing a character on the record medium.

8. The system as recited in claim 7 wherein:

said reversible timing means comprises a capacitor;

said means for initiating operation of said timing means comprises means for charging said capacitor from a reference potential; said means for stopping operation of said timing means comprises means for disabling said charging means; and

said means for operating said timing means in reverse comprises means for discharging said capacitor to said reference potential.

9. The system as set forth in claim 8 wherein said type carrier comprises a constantly rotating type wheel.

10. The system as recitedin claim 8 wherein the means operable when saidltiming means returns to its original state for printing one of the characters comprises: a differential amplifier connected across said capacitor and constructed and arranged to generate a signal transition when said capacitor discharges to said reference potential.

11. The system as recited in claim 7 wherein said -signalgenerating circuit comprises:

a moving code carrier driven at substantially the same velocity as said type carrier and having coded indicia thereon, a plurality of said indicia representing the different characters on said type carrier and spaced apart a distance corresponding to the distance between characters on said type carrier, another of said indicia located at a reference point on said code carrier;

means for sensing said indicia;

decode means connected to said sensing means for generating a SYNC pulse when said reference indicia is sensed by said sensing means, a SYNC pulse occuring at the beginning ofeach said signal of variable time interval;

means for supplying a coded input character; and

comparison means connected to said sensing means and said input means for generating a PRINT pulse when said input character matches one of said character representing indicia, a PRINT pulse occurring at the end of each said signal of variable time interval.

12. The system as recited in claim 11 wherein said signalgenerating circuit is located remote from said type carrier and is connected to the remainder of said control system by a transmission line.

13. The system as recited in claim 11 wherein said code carrier and said type carrier are driven by synchronous motors connected to the same power supply network.

14. The system as set forth in claim 7 wherein the HOME pulse precedes the end of said signal of variable time interval.

15. The system as set forth in claim 7 wherein the end of said signal of variable time interval precedes the HOME pulse. 

1. A system for controlling a printer including a moving type carrier, a record medium adjacent said carrier and a print hammer selectively operable to drive said carrier and said record medium together to print a character, said system comprising: a circuit for generating a character-representing signal, said circuit including means for generating SYNC pulses, at least some of which are followed by a PRINT pulse, the time interval between a SYNC and a following PRINT pulse representing a character with different time intervals representing different characters; means for generating a HOME pulse each time a full type font on said carrier passes said hammer; a capacitor; means for charging said capacitor at a predetermined constant rate; means responsive to a SYNC pulse to initiate charging of said capacitor at said constant rate; means response to the first-to-occur of the HOME and PRINT pulses immediately following said SYNC pulse for arresting said charging operation; means for maintaining the amount of charge on said capacitor substantially constant at the value of charge which said capacitor had at the time charging was arrested; means for discharging said capacitor at said predetermined constant rate; means responsive to the last-to-occur of said HOME and PRINT pulses for rendering said means for maintaining ineffective and initiating discharge of said capacitor; and means operable upon completion of said discharge to operate said hammer to print a character.
 2. The system set forth in claim 1 wherein said type carrier moves at a substantially constant velocity.
 3. The system set forth in claim 2 wherein said type carrier comprises a constantly rotating type wheel.
 4. The system set forth in claim 1 wherein said circuit means comprises: a moving code carrier driven at substantially the same velocity as said type carrier and having coded indicia thereon, a plurality of said indicia representing the different characters on said type carrier and being spaced apart a distance corresponding to the distance between characters on said type carrier another of said indicia located at a reference point on said code carrier; means for sensing said indicia; decode means connected to said sensing means for generating a SYNC pulse when said reference indicia is sensed by said sensing means; means for supplying a coded input character; and comparison means connected to said sensing means and said input means for generating a PRINT pulse when said input character matches one of said character-representing indicia.
 5. The system set forth in claim 4 wherein said circuit means is located remote from said type carrier and is connected to the remainder of said control system by a transmission line.
 6. The system set forth in claim 4 wherein said code carrier and said type carrier are driven by synchronous motors connected in the same power supply network.
 7. A system for controlling a printer wherein type characters in a font on a moving type carrier are selectively imprinted on a record medium adjacent the carrier, the system comprising: a circuit for generating a signal of variable time interval for representing characters to be printed, different characters being represented by signals having different time intervals; means for generating a HOME pulse each time a full type font on said carrier passes said record medium; reversible timing means; means for initiating operation of said timing means in one direction at a constant rate in response to the beginning of a said signaL of variable time interval; means for stopping operation of said timing means in response to the first-to-occur of the end of said signal of variable time interval or a HOME pulse; means for maintaining said timing means in the condition it was in at the time it was stopped; means for operating said timing means in a reverse direction at said constant rate such that it returns to its original state in response to the last-to-occur of a HOME pulse or the end of said signal of variable time interval; and means operable in response to said timing means returning to its original state for printing a character on the record medium.
 8. The system as recited in claim 7 wherein: said reversible timing means comprises a capacitor; said means for initiating operation of said timing means comprises means for charging said capacitor from a reference potential; said means for stopping operation of said timing means comprises means for disabling said charging means; and said means for operating said timing means in reverse comprises means for discharging said capacitor to said reference potential.
 9. The system as set forth in claim 8 wherein said type carrier comprises a constantly rotating type wheel.
 10. The system as recited in claim 8 wherein the means operable when said timing means returns to its original state for printing one of the characters comprises: a differential amplifier connected across said capacitor and constructed and arranged to generate a signal transition when said capacitor discharges to said reference potential.
 11. The system as recited in claim 7 wherein said signal-generating circuit comprises: a moving code carrier driven at substantially the same velocity as said type carrier and having coded indicia thereon, a plurality of said indicia representing the different characters on said type carrier and spaced apart a distance corresponding to the distance between characters on said type carrier, another of said indicia located at a reference point on said code carrier; means for sensing said indicia; decode means connected to said sensing means for generating a SYNC pulse when said reference indicia is sensed by said sensing means, a SYNC pulse occuring at the beginning of each said signal of variable time interval; means for supplying a coded input character; and comparison means connected to said sensing means and said input means for generating a PRINT pulse when said input character matches one of said character representing indicia, a PRINT pulse occurring at the end of each said signal of variable time interval.
 12. The system as recited in claim 11 wherein said signal-generating circuit is located remote from said type carrier and is connected to the remainder of said control system by a transmission line.
 13. The system as recited in claim 11 wherein said code carrier and said type carrier are driven by synchronous motors connected to the same power supply network.
 14. The system as set forth in claim 7 wherein the HOME pulse precedes the end of said signal of variable time interval.
 15. The system as set forth in claim 7 wherein the end of said signal of variable time interval precedes the HOME pulse. 